Update openfe_gromacs/protocols/gromacs_md/md_methods.py

Co-authored-by: Irfan Alibay <IAlibay@users.noreply.github.com>

openfe_gromacs/protocols/gromacs_md/md_methods.py

@@ -169,8 +169,8 @@ def _get_filenames(self, file_type) -> list[pathlib.Path]:
             for pus in self.data.values()
             if "GromacsMDRunUnit" in pus[0].source_key and file_type in pus[0].outputs
         ]
-        if len(file_paths) == 0:
-            file_paths = None
+        if not file_paths:
+            return None
 
         return file_paths